Field of the Invention
The present invention relates to an impedance converter circuit for a condenser microphone which utilizes an electron tube (vacuum tube).
Description of the Related Art
A condenser microphone generates an audio signal on the basis of a change in capacitance between a diaphragm and a fixed electrode which face each other.
That is, a condenser microphone wherein a diaphragm is located to face a fixed electrode has capacitance of around several tens of pF, and consequently the output impedance becomes extremely high. Such a condenser microphone therefore has a configuration such that an audio signal is extracted via an impedance converter circuit.
For such a condenser microphone, an impedance converter circuit which includes a field effect transistor (FET) or an electron tube (vacuum tube) is used. In particular, for a condenser microphone to be used for sound collection in a studio, a product is provided which uses an electron tube in an impedance converter circuit in order to improve the sound quality.
Impedance converter circuits which use electron tubes include a current amplifier circuit, which is called a plate grounded type or a cathode follower circuit, and a voltage amplifier circuit of a cathode grounded type.
In the case where a cathode follower circuit is used as an impedance converter circuit, it is generally known that a tone close to a condenser microphone which uses an FET described above is obtained.
On the other hand, a condenser microphone which uses a voltage amplifier circuit described above in an impedance converter circuit has a unique tone referred to as “vacuum tube like sound”, which is different from that of a cathode follower circuit. Such a tone still remains popular.
The former cathode follower circuit provides a wide dynamic range (range from a noise level to a level where distortion occurs). However, distortion occurs in the latter voltage amplifier circuit at a lower signal level than a cathode follower circuit. Therefore, a maximum allowable input sound pressure level as a microphone lowers when a voltage amplifier circuit is used in an impedance converter circuit. Accordingly, it is desired to use a voltage amplifier circuit including a vacuum tube as an impedance converter circuit for a condenser microphone so as to secure a wide dynamic range.
Now, in order to widen the dynamic range of the impedance converter circuit which uses a voltage amplifier circuit, means for applying negative feedback can be employed. That is, by connecting a condenser between a plate and a grid of an inverting amplifier which uses an electron tube, a signal generated by the plate can be fed back to the grid since a signal source is of a capacitance type (condenser).
This is called PG feedback, and impedance converter circuits wherein such PG feedback is conducted are disclosed in Japanese Patent No. 3,890,301 and Japanese Patent No. 4,426,902, for example.
FIG. 1 illustrates an example of an impedance converter circuit wherein PG feedback is conducted. In this example, a condenser Cp is connected as a feedback element between a plate and a grid of a voltage amplifier tube denoted by T1.
It is to be noted that the entire circuit configuration of the example illustrated in FIG. 1 excluding the configuration of a feedback circuit is the same as that of an embodiment according to the present invention which will be described below. Accordingly, detailed description on the entire circuit will be given below with reference to FIG. 2.
The plate of the electron tube T1 which constitutes a voltage amplifier has high impedance. Therefore, there is a limit on endeavor to increase a feedback quantity with the configuration of the PG feedback circuit illustrated in FIG. 1 in order to widen the above-described dynamic range.
That is, even when a feedback element having low impedance is used to increase the feedback quantity, a sufficient feedback quantity cannot be obtained since this becomes equivalent to series connection of impedance of the feedback element Cp with output impedance of the plate of the electron tube T1. Accordingly, a limit arises in circuit designing.
In order to solve such a problem, it is required to connect a feedback element with a signal source having low output impedance.